So far as is known, the Thomas Hill arithmometer of 1857 was the first key-driven calculating machine invented in the United States. The knob-like keys on the flat plate and the two wheels were connected by a ratchet arrangement. The operator used a combination of wheel setting and key punching to add, subtract, multiply or divide. The answer appeared through a viewing window mounted on the left-hand wheel. This invention anticipated by 30 years the mass-produced key-driven machines of the late 19th century.